Text Processing with Sed

Intermediate Linux Fundamentals
sedstream editorregex substitution

Objective

Use sed as a stream editor to perform substitution, deletion, insertion, and line selection on text files and streams.

Tools & Technologies

  • sed
  • stream editor
  • regex substitution

Key Commands

sed 's/old/new/g' file
sed -i.bak 's/foo/bar/g' file
sed -n '5,10p' file
sed '/^#/d' config

Architecture Overview

flowchart LR INPUT[Input\nline by line] --> READ[Read line\nto pattern space] READ --> MATCH{Pattern\nmatches?} MATCH -->|yes| ACTION[Apply\ncommand] MATCH -->|no| OUTPUT ACTION --> OUTPUT[Print to\nstdout] OUTPUT --> READ OUTPUT -->|EOF| DONE[Done] style INPUT fill:#1a1a2e,stroke:#00d4ff,color:#e0e0e0 style ACTION fill:#1a1a2e,stroke:#00ff88,color:#e0e0e0

Step-by-Step Process

01
Substitution

The s command is the most-used sed operation. Format: s/find/replace/flags

sed 's/foo/bar/' file       # first match per line
sed 's/foo/bar/g' file      # all matches (global)
sed 's/foo/bar/2' file      # second match only
sed 's/foo/bar/gi' file     # global + case insensitive

# In-place edit with backup
sed -i.bak 's/old/new/g' file

# Remove trailing whitespace
sed 's/[[:space:]]*$//' file
02
Line Selection

Apply commands only to specific lines using line numbers, ranges, or patterns.

sed -n '5p' file           # print line 5 only
sed -n '5,10p' file        # print lines 5-10
sed '1d' file              # delete line 1
sed '1,5d' file            # delete lines 1-5
sed '/^#/d' config         # delete comment lines
sed -n '/START/,/END/p'    # print between patterns
03
Insert & Append

Add lines before or after matched patterns.

# Insert line before pattern
sed '/pattern/i\New line before' file
# Append line after pattern
sed '/pattern/a\New line after' file
# Insert at line number
sed '3i\Inserted at line 3' file
04
Practical Use Cases

Real sed one-liners for config management and data transformation.

# Comment out a line
sed 's/^PermitRootLogin/# &/' /etc/ssh/sshd_config

# Uncomment a line
sed 's/^# *PasswordAuthentication/PasswordAuthentication/' sshd_config

# Remove blank lines
sed '/^$/d' file

# Add line numbers
sed = file | sed 'N;s/\n/\t/'

Challenges & Solutions

  • sed -i without backup is destructive — always use -i.bak first
  • GNU sed -i differs from BSD sed -i — macOS requires -i ''

Key Takeaways

  • sed is ideal for single-pass transformations; awk is better for structured data
  • Test sed patterns without -i first to verify output